1. Field of the Invention
This invention relates generally to radial seals. More particularly, the invention relates to a method for making an improved fluoropolymer radial seal, such as a radial shaft seal, that is bonded directly to an elastomeric casing layer.
2. Description of the Prior Art
Radial shaft seals that are designed for use in sealing the main rotating shaft of vehicle air conditioner compressors, superchargers, power steering pumps, and engine crankshafts may utilize multiple sealing elements designed such that a first sealing element facing the fluid or gas to be sealed is an elastomer, such as a natural or synthetic rubber. The elastomer generally has sufficient flexibility and resilience to provide a seal against the shaft. A second stiffer, lower friction, and more chemically resistant sealing element is generally positioned behind and in tandem with the elastomeric seal such that an axial gap is provided between the sealing edge of the stiffer wear-resistant seal and the back sealing edge of the more resilient elastomeric sealing element. The second sealing element is generally made from a fluoropolymer, such as polytetrafluoroethylene (PTFE), or a filled PTFE material which incorporates one or more known filler materials to control the mechanical, tribological or other properties of the PTFE.
Generally in the art, the elements of such seal structures have been typically assembled together and then are clamped together in a unit using a crimping process. In such a process, a rubber element and the PTFE component are crimped between two rigid casings to form a seal. The PTFE component is also typically crimped between the rubber element and one of the rigid casings. It is known in the art to utilize a flat PTFE washer or preformed conical-shaped structure that is bonded or clamped to form the overall seal.
Other radial shaft seal designs have also been proposed which do not utilize crimping or clamping of the elastomer and PTFE component into a rigid casing, but rather utilize a metal casing to which the PTFE sealing element is attached by molding an elastomeric member to both the PTFE sealing element and the metal casing. In such designs, the PTFE element may be used only as a bearing member to support and control the load of the elastomeric sealing element, such that the sealing function is entirely performed by the elastomeric sealing element. An example of such a seal configuration is shown in U.S. Pat. No. 4,274,641 to Cather. In this configuration the PTFE bearing member and the elastomeric sealing lip are bonded in tandem and are both in contact with the shaft surface. Similarly, in U.S. Pat. No. 6,428,013 to Johnston et al. several seal designs are disclosed where both the PTFE sealing element and elastomeric element are in contact with the shaft surface on which sealing is to be affected.
Still other seal designs have also been proposed which do not incorporate an elastomeric sealing element and which rely entirely on a PTFE sealing element to provide the fluid seal. One such radial shaft seal design is described in U.S. Pat. No. 4,650,196 to Bucher et al. In Bucher et al., the PTFE element is bonded over a portion of its length to an elastomeric casing which is in turn bonded to a rigid casing. Similarly, in Johnston et al. several seal designs which incorporate a PTFE sealing element as the primary sealing element are disclosed.
One limitation of the related art radial shaft designs, such as those described above, is that the PTFE sealing element does not seal along its entire length. For example, in the designs of Johnston et al. the PTFE sealing element is not in contact with the shaft along its entire length. This is also the case for the PTFE member of Bucher et al. leading to a sub-optimal use of the available PTFE sealing material. Furthermore, these radial seal designs also provide limited control of the sealing pressure applied either by the PTFE sealing element itself to the shaft or other sealing surface, or else by the combination of the elastomeric casing and the PTFE sealing element to the shaft or other sealing surface because of the limited contact area of the PTFE. In addition to the limitations noted above, related art radial shaft seal designs also have known limitations with respect to installation of the seals onto the shaft or other member to be sealed. Many of the known designs where the PTFE lip is the primary sealing lip have the free end of the radial sealing lip facing the fluid side, usually the oil side, of the sealed region. These configurations are known to be difficult to install onto circular shafts and the like, necessitating the use of special fixtures and installation tools, and special assembly precautions or methods to assemble such seals on shafts so as to avoid nicking or otherwise damaging the surface of the PTFE material, and thus destroying the functionality of the seals. Fluoropolymer sealing materials, such as PTFE, are known to be very susceptible to nicking or other surface damage to the sealing surface which can compromise their ability to seal effectively. Reverse lay down configurations of the PTFE sealing element, where the free end of the sealing element faces away from the oil side of the installation, have been proposed, such as in Johnston et al., in order to enhance the ability to install such seals and lessen the susceptibility to nicking, inverse folding, or creasing during installation. However, such seal configurations are still believed to be subject to other limitations, such as those described above.